It is known that aluminum compounds can function as coordination driers in paints and they have been used as replacements for lead driers because of their relatively low toxicity. The preferred aluminum compounds are the alkoxides such as aluminum isopropoxide, higher alkoxides, substituted alkoxides, and the oxo-aluminum compounds.
Oxo-aluminum compounds are generally obtained by reacting optionally substituted aluminum alkoxides with water, monocarboxylic acids, alcohols and/or phenols in the manner disclosed in several prior references such as for example British Patent Specification Nos. 806113, 825878 and 907558. British Specification No. 1001837 discloses oxo-compounds prepared by methods analogous to those described in prior specifications which contain in addition to aluminum atoms, one or more selected hetero atoms such as copper, magnesium, zinc, cadmium, lead, etc.
To effect the condensation reaction one mole of water is used for each two alkoxide groups present and the reaction may be represented as involving the following stages: ##STR1##
On heating this partially hydrolyzed compound under distillation conditions, inter- and intra-molecular condensation will result in condensation which may be represented as follows: ##STR2## and with further heating: ##STR3##
However, in surface coating applications, the usefulness of these aluminum compounds is limited by their excessive reactivity with free carboxyl groups. Other potentially reactive groups are hydroperoxyl, hydroxyl and active methylene groups which may be available in the resin medium but in practice are found to be of lower reactivity. This reaction may be represented as follows for free carboxyl groups: ##STR4##
Because of this reactivity, association occurs and tends to increase the molecular weight and hence the viscosity of the medium. This can be countered by dilution with more solvent but the resulting decrease in the solids content causes a diminution in the binding properties and durability of the medium. Therefore, the successful use of such aluminum compounds as driers has been mainly limited to applications such as in printing inks and surface coating applications in which a high viscosity or low solids content is not a disadvantage. The usefulness of aluminum compounds containing alkoxide substituents may also be impaired by the readiness with which such compounds can be hydrolyzed by atmospheric and other sources of water with which they may come into contact. This has also imposed limitations on the applications for which some aluminum compounds and their derivatives are suitable.
British Patent Specification No. 772480 discloses aluminum compounds having the oxo-linkage and alternatively carboxyl and alkoxy substituents on the aluminum atoms which are formed by reacting aluminum alcoholates with a mono-basic organic acid or acid amide substance. It is proposed that the products may be used in the lacquer and varnish industries. However, these products would not be eminently suitable as driers in surface coating formulations containing drying oils or resins as the number of alkoxide groups present on the aluminum compound would make the products particularly sensitive to hydrolysis by atmospheric moisture.
Similarly, British Specification No. 980,110 discloses oxo-aluminum compounds having alkoxide group substituents which would make the products sensitive to hydrolysis and therefore affect stability of a surface coating composition containing a drying oil or resin if incorporated therein.
British Specification No. 767,585 discloses the use of compounds disclosed in British Specification No. 772480 in rendering fibrous materials water repellent.
Various proposals have been made to counter the increase in structure of a paint or varnish when certain organo aluminum compounds are added to media containing drying oils, alkyd resins or other resinous bodies with which the aluminum compounds can react.
U.S. Pat. Nos. 4,090,886 and 4,264,370 disclose a method for achieving the desirable characteristic of storage stability in air-drying compositions comprising an aluminum compound and a reactive paint medium in a mixture ratio which would normally prove to be unstable and cause gelation in the container before being applied to a surface. This is achieved by adding at least one mole, per mole of the aluminum compound, of certain labile monofunctional reactants, preferably a monohydroxy compound such as a lower alkanol. The labile monofunctional reactants will react preferentially to impede the complex association which occurs when aluminum compounds are added to drying oils or paint media containing reactive groups such as hydroxyl and carboxyl. It is assumed that in the presence of the labile reactant, an equilibrium is established which is disturbed when the film is applied to a surface whereby the labile reactant is released by hydrolysis or by its own volatility. The equilibrium may be represented as follows: ##STR5##
Oxo-aluminum compounds, however, do not readily complex with weak hydroxyl groups on alkyd resins. They do react slowly with the stronger carboxyl groups resulting in the undesirable formation of hydroxy-aluminum disoaps as shown in the foregoing, which cause surface coating media to thicken on storage.
Other patents relating to organoaluminum compounds are U.S. Pat. Nos. 2,744,074, 2,979,497, 3,198,332, 3,487,097, 4,055,634, 4,075,178, 4,132,724 and British Pat. Nos. 809,309, 888,666, 1,544,405 and Czech Pat. No. 85300. See also Mehrotra, "Aluminum Alkoxides," J. Indian Chem. Soc., Vol. 30, No. 9, pages 585-591 (1953).
I have now found oxo-aluminum complexes which are suitable for use as coordination driers in surface coatings based on synthetic resins which are substantially free from the attendant disadvantages just described.